The art of lithographic printing is based upon the immiscibility of oil and water, wherein an ink is preferentially retained by the image areas and the water or a fountain solution is preferentially retained by the non-image areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and other materials. Commonly, in offset printing, the ink is transferred to an intermediate material referred to as a blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A widely used type of lithographic printing plate precursor has an image-forming layer applied on a substrate. The layer may respond to light under exposure and become soluble so that it is removed during development. Such a plate is referred to as a positive working plate. Conversely, when an image-forming layer is exposed to light and is hardened so that it remains on the substrate during development, such a plate is referred to as a negative working printing plate. In both instances, the portion including the remaining image-forming layer is the image area and is ink-receptive or oleophilic. The portion where the image-forming layer has been removed is the non-image area or background and is water-receptive or hydrophilic.
Since the non-image area or background of the lithographic printing plate is the portion where the image-forming layer has been removed during development, the substrate under the image-forming layer is exposed. Although the substrate used for the lithographic printing plate precursor is usually a metal plate, a plastic film or a composite material, the material per se has no hydrophilicity. When the substrate is made of the metal plate, it is usually subjected to a roughening treatment so as to impart water retentivity. An interlayer capable of imparting hydrophilicity is usually provided between the substrate and the image-forming layer.
There have conventionally been used, as the material capable of imparting hydrophilicity of the interlayer, for example, polyvinylphosphonic acid, polyacrylic acid, polyacrylamide, silicate, zirconate and titanate.
Japanese Unexamined Patent Publication (Kokai) No. 06-340720 describes that a thermally crosslinkable hydrophilic copolymer including polymer chains composed of four kinds of monomer units having an acid side group and a base side group is used to hydrophilized a lithographic printing plate. The publication describes that acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, a maleic acid monoester, a fumaric acid monoester, an itaconic acid monoester, vinylsulfonic acid, vinylbenzoic acid or vinylphosphonic acid is used as the acid side group, while dimethylaminoethyl acrylate or vinyl pyridine is used as the base side group.
Japanese Unexamined Patent Publication (Kokai) No. 2008-511858 describes an interlayer of a lithographic printing plate, which comprises a copolymer including a polyalkylene oxide unit and a unit derived from vinylphosphonic acid and optionally a (meth)acrylamide unit.
International Publication No. WO 2005/010613 pamphlet describes that a photosensitive lithographic printing plate having an aluminum substrate which is treated with an aqueous alkali metal salt solution and is also treated with a polyvinylphosphonic acid solution or an aqueous polymer solution of an unsaturated acid having a carboxyl group.
It has been known that it is possible to obtain a lithographic printing plate capable of obtaining printed matter, which is free from stain during printing, by pre-treating the substrate with such a compound. However, in the printing plate subjected to a hydropholization treatment, various stains may be generated according to materials (paper, ink, fountain solution, etc.) to be used and printing conditions.
Recently, a copolymer of vinylphosphonic acid and (meth)acrylic acid has often been used as the material of the interlayer. However, the interlayer made of this copolymer is inferior in ink cleanup characteristics. It is known that a carboxyl group of (meth)acrylic acid is converted into an acid form when an acidic fountain solution is used, thus causing a problem such as restart toning.
It was found that a lithographic printing plate comprising an interlayer made of a copolymer of vinylphosphonic acid and (meth)acrylic acid causes a problem such as restart toning during printing using a synthetic paper containing a certain amount of an antistatic agent on a surface, for example, Yupo® synthetic paper (manufactured by Yopo Corporation). It is believed that reason for restart toning is that the antistatic agent is adsorbed on the interlayer which is made of a vinylphosphonic acid-(meth)acrylic acid copolymer during printing.
Japanese Unexamined Patent Publication (Kokai) No. 07-001853 describes, as a more effect interlayer which imparts stable hydrophilicity to an aluminum substrate and also imparts satisfactory adhesion for a negative working or positive working radiation-sensitive film to be subsequently formed, an interlayer containing a copolymer of 40 to 90 mol % of vinylphosphonic acid and 10 to 60 mol % of acrylamide. It was found that this interlayer solves the problem of tinting, toning or scumming due to poor adhesion of the remaining coating film during development, but does not solve the problem of restart toning, in particular restart toning during printing on a synthetic paper containing an antistatic agent.